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Old 09-05-2019, 10:58 AM
Autoputzer Autoputzer is offline
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Location: NW Floriduh
Join Date: Mar 2014
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Mein Auto: 2014 535i + 2018 X3 30i
Originally Posted by RPsX5d View Post
I wonder what (if anything) would cause the K-factor effect to change? i.e. once set to match manual calculation, why would the two readings drift apart?

For several years, my indicated reading and manually calculated values pretty much matched . . . about 22.5 mpg. This value held for a very long time (several years). Then suddenly the indicated dropped to about 18 mpg, and as I indicated above I didn't repeat the manual calculation . . . I assumed something was going wrong, however, no fault codes.

I am now rechecking indicated vs. manual . . . two tanks done . . . both showing the indicated around 18 mpg . . . manual about 22.5 mpg, same as what it has been for a very long time.

One possibility is after resetting the K-factor your drive profile has changed substantially, re-adjust the K-factor to match the new drive profile. In my case, the drive profile did not change, same fuel station, etc . . .
My guess is that variations in fuel pressure is the biggest factor, followed variations in the injectors and deposits building up in the injectors restricting flow. Our current two BMW's OBC MPG/actual MPG is slowly drifting up, suggesting that fuel flow (and maybe fuel pressure) is going down slightly with time. For diesels, fuel viscosity changes with temperature could be significant.
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